1. Field of the Invention
The present invention relates generally to the modulation of a semiconductor laser and more specifically to a system and method for modulating a distributed feedback (DFB) or a distributed Bragg reflector (DBR) semiconductor laser without undesirable, thermally induced wavelength drift.
2. Background
Lasers find particular utility in display technologies such as computer screens and televisions and the like. In such displays, lasers generate the three primary colors (red, blue and green) which are mixed in various combinations to provide a color image. The output beam of each laser can be raster-scanned across the screen or can be stationary and employed to illuminate pixels forming an image (e.g., a motion picture film or spatial light modulator containing an image). The ability of a laser to provide a beam having excellent brightness characteristics leads to projectors that are more efficient and better performing compared to projectors utilizing incandescent bulbs.
Semiconductor lasers such as DBR and DFB lasers are particularly useful for laser-based displays as their output beams can be efficiently converted to useful primary color wavelengths. For example, a 1060 nm DBR or DFB semiconductor laser tuned to a spectral center of a second-harmonic-generation (SHG) device such as a non-linear crystal may be used to generate a 530 nm beam. This provides a low-cost, compact and efficient non-linear source of green light.
Generally, for technologies involving video displays, the optical power used to generate the intensity of a primary color needs to be modulated at a fundamental frequency of approximately 50 MHz and with an extinction ratio (i.e., the ratio of highest to lowest optical power) of approximately 40 dB. The practical and economical achievement of such a combination of high modulation speed and large extinction ratio has proven difficult in the prior art.
One prior art technique for achieving a fast modulation and a large extinction-ratio in the combination of a semiconductor laser and second harmonic generator (SHG) is to rapidly modulate the wavelength of the output beam of the semiconductor laser. Such a modulation technique exploits the fact that non-linear SHG devices are typically capable of converting only a very narrow range of incoming laser wavelengths into longer wavelength light. In operation, the wavelength of the semiconductor laser beam rapidly scans across the narrow spectral width of a non-linear SHG device to produce the necessary intensity modulation. For example, if maximum green power is needed, the wavelength is tuned to the center wavelength of the non-linear crystal while, if zero green power is needed 10 ns later, the wavelength is tuned to one side or the other of the center wavelength that is outside of the spectral width of the SHG device.
FIG. 1A schematically illustrates a conventional DBR semiconductor laser 100 and a second harmonic generation (SHG) device 150. The DBR semiconductor laser 100 includes a DBR portion 110, a phase portion 120 and a gain portion 130. The gain portion 130, when injected with a continuous wave (CW) current, generates continuous optical power for the laser. The current injected into the DBR portion 110 makes large changes to wavelengths output from the laser and the current into the phase portion 120 makes small changes to the wavelength of the beam output of the laser. The SHG device 150 receives the beam produced by the semiconductor laser 100, whose output intensity of the converted wavelength (green, for example) depends upon alignment of the DBR laser wavelength and the SHG device's spectral center. The beam output from the SHG device 150 is then directed to an output such as a display screen.
FIG. 1B schematically illustrates a conventional DFB semiconductor laser 160 and an SHG device 170. The current injected into the DFB semiconductor laser 160 controls the output intensity from the laser, and the SHG device 170 receives the beam produced by the semiconductor laser 160. The output intensity of the converted wavelength (green, for example) depends upon the current input into the DFB semiconductor laser 160. The beam output from the SHG device 170 is then directed to an output such as a display screen.
The simplest way to rapidly tune the DBR semiconductor laser's output wavelength is by injecting modulated current into the DBR portion and phase portion of the DBR semiconductor laser 100 while keeping the gain-portion current continuous and constant. As illustrated in the chart provided in FIG. 2A, a video signal can require green light with an intensity of up to 100% within each bit period of the signal. The bit period width is the inverse of the system frequency, for example, the resident time of each pixel of a raster scan on a display screen. For the example shown in FIG. 2A, an intensity of 100% is the brightest possible signal while 0% is dark. Thus, as illustrated in FIG. 2A, the video intensity required for the first bit period is 100%, the intensity reduces to 0% for the second bit period and is increased to 40% for the third bit period.
With conventional systems, the current injected into the DBR portion 110 is pulse width modulated based on the required intensity in each bit period. That is, the duration within one bit period in which the current is “on” is proportional to the intensity of the video signal in that bit period (shown in the first waveform from the top of FIG. 2A). Ideally, the wavelength of the output of a DBR semiconductor laser is shifted based on the carrier-induced effect and output to the SHG device 150 (shown in the second waveform from the top of FIG. 2A). The SHG device 150, based upon the received beam, outputs a converted beam having an ideal intensity signal for display, as illustrated in FIG. 2A. However, the simple scheme described above ignores the possible adverse thermal effect that the injection of current into the laser causes.
Similarly, for a DFB laser, the current injected into the DFB laser 160 is pulse width modulated based on the required intensity in each bit period, as shown in FIG. 2B. Ideally, the wavelength of a DFB semiconductor laser is constant. However, the wavelength changes according to temperature, as shown in FIG. 2B.
Generally, current injection into the DBR portion of a DBR semiconductor laser generates two effects within the DBR semiconductor laser. First, a carrier effect is generated that provides more carriers in the portion increasing carrier density and reducing the refractive index within the laser. As a result, a shorter wavelength beam is generated. Current injection also causes a heating effect which causes the temperature of the DBR semiconductor laser device to rise. More specifically, currents higher than zero rise the temperature in the DBR semiconductor laser, thereby increasing the refractive indices, which tend to generate a longer wavelength beam. The collective wavelength shift is produced by the combined effect of the carrier effect and thermal effect. For large current values that are needed to achieve large wavelength shift, the temperature rise is severe enough to reduce and sometimes completely reverse the carrier-induced wavelength shift. For the case of a DFB laser, the current injection causes DFB laser temperature to change and therefore red shifts the wavelength (i.e. shifts to a longer wavelength).
Another feature of current induced thermal effect is that it provides a slow wavelength modulation process. The thermal effect, which causes the temperature of the laser to increase, has μs-to ms response time compared to the carrier effect that has ns response time. The degree of thermal effect also depends upon the current amplitude and the heat sinking conditions associated with the laser. The slow response of the thermal effect means that the wavelength does not change for pulse widths much smaller than 1 μs, for example, 20 ns. Slow thermal effect results in an undesirable patterning effect because the average heating depends upon the width of pulses and therefore on the pattern of the video signal. In other words, the DBR or DFB semiconductor laser wavelength at a particular bit of the video signal depends on the history of the previous bits of data.
FIGS. 2A and 2B illustrate the effect that the injection of current, and the resulting increase in temperature, can have on the operation of the laser. The adverse effects resulting from a temperature rise in the laser are also shown in the charts of FIGS. 2A and 2B. Specifically, in FIG. 2A, when injection current is applied to the DBR portion 110 of the DBR semiconductor laser 100, and the current is constantly on, the DBR temperature rises as shown by the DBR temperature waveform in FIG. 2A. As a result, the actual DBR wavelength waveform provided from the laser to the SHG device 150 will be distorted, and the resulting output from the SHG device 150 will also be distorted, and the required intensity of the original video signal is not achieved at the output of the SHG device 150. These same adverse effects accompany the operation of DFB semiconductor lasers, shown in FIG. 2B. The actual DFB wavelength provided from the laser 160 to the SHG device 170 will be distorted, and the resulting output from the SHG device 170 will also be distorted.
Another example of modulating a semiconductor laser output is by modulating the output intensity instead of the wavelength that was discussed in previous paragraph. A modulating current is applied to the entire length of a DFB laser or the gain section of a DBR laser. High-level amplitude of the modulating current results in high output intensity and lower amplitude results in reduced output intensity. The laser wavelength is ideally maintained constant. For the case of an optical system consisting of a DBR or a DFB laser and a SHG, the laser output wavelength is desired to be constant and aligned to the spectral center of a SHG.
Accordingly, what is needed is a way to modulate the output of a semiconductor laser, or the combination of such a laser and an SHG, without the creation of thermal drift that in turn creates an undesirable thermal patterning effect. Ideally, such a technique should be compatible with short pulse widths on the order of 10 ns and an extinction ratio of 40 db so that a high bit rate of information can be transmitted. Finally such a modulation technique should be easy and inexpensive to implement, and compatible not only with DBR lasers, but lower-cost DFB lasers as well.